Molecular-dynamics simulations of ion size effects on the fluid structure of aqueous electrolyte systems between charged model electrodes

The effect of ion size on the structure of aqueous electrolyte solutions between charged nonpolarizable surfaces or electrodes is investigated using molecular-dynamics simulations of discrete water molecules and ions confined to a slab geometry. Long-range intermolecular Coulombic interactions are calculated using the particle–particle–particle–mesh method with a modification to account for the slab geometry. Density distribution and potential profiles are reported for 1 M aqueous electrolyte solutions with ±0.1 C/m2 electrode surface charge at the electrode surfaces. Five different models for the ions are studied. The models can be characterized as (1) ions of equal size, (2) smaller cations, (3) larger anions, (4) smaller cations and larger anions, and (5) ions representing aqueous NaCl. Compared to the equal-size ion reference case, smaller cation size decreases the contact adsorption at the cathode, but interestingly anion size tends to moderate this effect somewhat. Whereas there is no contact adsorption of anions when the ions are the same size, an increase in anion size causes significant replacement of adsorbed water molecules at the anode with anions. In the case studied here, the larger anion size produced a 20 fold increase in localized density at the electrode surface. The electrostatic potential profile tends to be relatively insensitive to the changes in ion size.